[oe] linux-libc-headers version (reloaded)

[Andreas Oberritter]

On 02/24/2011 02:30 PM, Steffen Sledz wrote:
> On 02/18/2011 04:30 PM, Khem Raj wrote:
>> On Fri, Feb 18, 2011 at 1:55 AM, Steffen Sledz <sledz at dresearch.de> wrote:
>>> Am 15.02.2011 15:50, schrieb Steffen Sledz:
>>>> Am 15.02.2011 15:12, schrieb Andreas Oberritter:
>>>>> On 02/15/2011 11:41 AM, Steffen Sledz wrote:
>>>>>> "Kernel headers are backwards compatible, but not forwards compatible.  This
>>>>>> means that a program built against a C library using older kernel headers
>>>>>> should run on a newer kernel (although it may not have access to new
>>>>>> features), but a program built against newer kernel headers may not work on an
>>>>>> older kernel."[2]
>>>>>
>>>>> Isn't this what the variable OLDEST_KERNEL is good for, when compiling
>>>>> glibc?
>>>>
>>>> If i'm right this goes to the --enable-kernel=VERSION configure option of glibc just to optimize the library.
>>>>
>>>> "the configure option --enable-kernel=X.Y.Z allows to strip out compatibility for kernel versions before X.Y.Z."
>>>>
>>>> Imho it is not legitimately to follow that glibc has compatibility code for all kernels greater or equal X.Y.Z.
>>>>
>>>> Another question is the handling in other libc implementations.
>>>>
>>>> And finally there are a lot of programs using userland kernel headers directly.
>>>
>>> Ping!
>>>
>>> If i interpret responses from Tom and Phil right they agree with me (or at least do not disagree). ;-)
>>>
>>> But i miss reactions from the distro maintainers (especially Ångström).
>>>
>>
>> I think we should make sure that linux version chosen for a build is
>> equal or newer than linux-libc-headers for that build. Another option
>> is that linux-libc-headers are driven out
>> of selected virtual/kernel too but this may be a bit clunky since it
>> would mean that
>> every machine will have them different and we share sysroots e.g. two
>> armv5te may use
>> same sysroot
> 
> I like to force the discussion/work/decision on this problem because we're one of the mourners (we're forced to use 2.6.24 kernel by out hardware vendor :( ).
> 
> I also see the multi-machine problem (the shared sysroot at build time and the feed problem too).
> 
> So what options do we (our Ångström) have?
> 
> (1) Do not support kernel older than 2.6.31 (which is the current LINUX_LIBC_HEADERS_VERSION).
> 
> (2) Set LINUX_LIBC_HEADERS_VERSION to 2.6.16 (which is the current OLDEST_KERNEL).
> 
> (3) Support machine specific distro incarnations (incl. special feeds).
> 
> May be some more. Option (1) would be really bad for us. I believe (2) would be bad for a lot of users because of a potential loss of functionality.

I'm still not convinced that requiring older headers is a good way to
go. If applications are using new syscalls directly, they need to handle
ENOSYS. If the applications already contain code to be compatible across
various kernel versions at compile time, then it won't be that hard to
change those applications to detect available interfaces at runtime.

IIRC, the kernel policy is to disallow incompatible changes to user
interfaces, so applications compiled against new headers generally don't
magically start to use new interfaces, unless their code gets adapted to
them.

Can you please shed some light on which kernel interfaces are actually
causing problems?

Btw.: The availability of syscalls not only depends on the kernel
version but on the kernel configuration, too. For example, if you
compile a kernel without CONFIG_EPOLL, epoll will still be available in
the kernel headers and will be found by autotools etc. But it will
alwyas return ENOSYS. We certainly can't provide matching
linux-libc-headers to avoid that.

Regards,
Andreas